HR: 14:40h
AN: GP22A-05 INVITED     [PDF]
TI: Origin of Lunar Crustal Magnetic Anomalies: Role of Impact Processes
AU: * Hood, L L
EM: lon@lpl.arizona.edu
AF: Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd., Tucson, AZ 85721-0092 United States
AB: A combination of orbital, surface, and returned sample measurements shows that impact processes have exerted an important influence on the production and distribution of crustal magnetic field sources on the Moon. Unlike the Earth and Mars, where oxidizing conditions have predominated, the main ferromagnetic carriers in lunar materials are reduced metallic iron particles. These ferromagnetic carriers are sparse in igneous materials such as mare basalt but are concentrated in impact-generated breccias and fines through reduction of pre-existing iron silicates by shock and/or heat. Consistent with these laboratory results, surface magnetometer measurements at the Apollo landing sites yielded the largest surface fields (exceeding 300 nT) in a region dominated by the Cayley Formation and the Descartes highlands, both considered by planetary geologists to represent impact basin ejecta materials. Orbital studies such as those conducted by Lunar Prospector have provided additional evidence that impact basin ejecta (as opposed to smaller crater ejecta) are the main sources of the orbital anomalies. On the geologically less complex near side, correlations of orbital anomalies have been found with visible exposures of the Cayley, Descartes highlands, and Fra Mauro Formations, all interpreted as primary and/or secondary basin ejecta materials. On a larger scale, a general inverse relationship between large impact basin locations and crustal magnetic field strength is obtained such that fields are weak over the basins but are strong near their antipodes. A model for explaining the stronger fields near basin antipodes has been partially developed theoretically and involves the interaction of impact-produced plasmas with an ambient large-scale magnetic field. Since the basins in question are all older than 3.8 Gyr, the data can be interpreted as indicating an early lunar core dynamo. More definitive tests of this hypothesis are possible using existing data and will be described.
DE: 5420 Impact phenomena (includes cratering)
DE: 5440 Magnetic fields and magnetism
DE: 6250 Moon (1221)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting